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Old 30-01-2012, 06:59   #16
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Re: Drilling New Holes

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Originally Posted by HopCar View Post
I've never tried this but a smart man once told me that the way to enlarge a hole like that, you needed two hole saws. One hole saw the size of the existing hole and one the size of the hole you need. You mount both saws on the same mandrel with the small saw inside the large saw. When you drill, the small saw acts as a guide keeping the larger saw centered around the existing hole. Has anybody actually tried this?
I do this all the time. Just the other week, in fact. Works perfectly, although you may need to pull the saw back and clean out the space once in a while depending on the material and thickness.

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Old 30-01-2012, 07:10   #17
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Re: Drilling New Holes

To enlarge a hole (as previously noted), use two hole saws on one arbor. Use one smaller hole saw the size of the original hole, and a larger hole saw the size of the new hole.

Install the two hole saws, the smaller inside the larger. The small one should protrude at least 1/2" beyond the larger one. Insert the smaller saw into the old hole, and carefully guide the saw through the hole. Keep the drill on a straight, steady course as the larger saw cuts the new hole around it.

See also ➥
Hole Saw Tips and Tricks

And:

Oops ArborEnlarge Holes With The Starrett Oops Arbor | Toolmonger

Hole Saw Alignment GuidesHole Saw Alignment Guide
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Old 30-01-2012, 14:27   #18
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Re: Drilling New Holes

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Originally Posted by GordMay View Post
To enlarge a hole (as previously noted), use two hole saws on one arbor. Use one smaller hole saw the size of the original hole, and a larger hole saw the size of the new hole.

Install the two hole saws, the smaller inside the larger. The small one should protrude at least 1/2" beyond the larger one. Insert the smaller saw into the old hole, and carefully guide the saw through the hole. Keep the drill on a straight, steady course as the larger saw cuts the new hole around it.

See also ➥ Hole Saw Tips and Tricks

And:

Oops ArborEnlarge Holes With The Starrett Oops Arbor | Toolmonger

Hole Saw Alignment GuidesHole Saw Alignment Guide
To me, the real advantage to doing it this way for this application is that it will help you to drill the hole plumb, which is the tricky part of drilling a hole for a 'ducer. If you use this method it will drill the hole at exactly the same angle as the original with minimal effort. This is important, if you just drill the hole square to the hull you will obviously have serious problems. Being even a little off of plumb can cause you to have to grind for a plumb fit on the 'ducer, which results in a sloppier fit and more potential for movement and leakage. I do a lot of this, because the newer plastic 'ducer fairings are a PITA to cut perfect, and they cost about $200 a pop. I seem to be the only one in the shop who doesn't ever cut it backwards. So I get to do most 'ducer installs in the yard. Careful measurement with the bevel gauge and a level is key.
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